One example of a motion detecting system for detecting the moving speed and direction of an object from imagery produced by an imaging device such as camera is described in published Japanese patent application JP 2003-67752. This published patent application shows a motion detecting system mounted in a vehicle and arranged to recognize a preceding vehicle and/or an object in the periphery. This system extracts, from an electronically produced source image, an edge of a preceding vehicle having luminance varying in accord with motion of the preceding vehicle, calculates an optical flow (a vector representing motion in the image) in accordance with changes in edge position in consecutive picture frames and calculates the speed of the preceding vehicle. The optical flow can be detected by using a correlation method of determining corresponding locations among frames and determining a motion vector.